Intracerebral inoculation of 4 day old dogs with Rous Sarcoma virus yields brain tumors with 100% incidence in 4 weeks. Tumor type can be specified by site of inoculation; previous work suggests that the gliomas derive from the primitive neuroectodermal cells of the subependymal plane. By studying this structure in the lateral wall of the lateral ventricles with correlative light and electronmicroscopy, at intervals from .12 to 24 hours after viral inoculation, we can: 1) evaluate the role of the subependymal plate as the cellular source of telencephalic gliomas, by means of radioisotopic labelling of glioblasts concomitant with their neoplastic transformation by the Rous sarcoma virus, 2) observe the early and sequential structural events of the neoplastic transformation and 3) trace the fate of the viral genome within the transformed neuro-glial cells by using labelled viral inocula and techniques to detect viral antigens.